Attempts to increase the efficiency of the IC engine through ultra-lean, fast burn, high compression ratio, have had limited success, principally because of the inability to operate at the high compression ratios needed for highest efficiency. In the case of Diesel engines, high compression ratio (CR) of over 13 to 1 have generally not been successful in increasing efficiency because of the higher friction and heat transfer losses associated with the high CR. That is, above a certain compression ratio, high friction and high heat losses offsets any gains in efficiency due to the higher CR, as pointed out by Komatsu in an SAE paper on the spark ignited Diesel. However, in the case of gasoline engines, when high octane fuel was available, compression ratios of 15 to 1 were used with lean burn to achieve 40% to 50% better fuel economy, as shown by Michael May with his fast burn, lean burn Fireball Engine, reported in a 1979 SAE paper No. 790386. Also, the Ricardo Engineers, England, had some success with their High Ratio Compact Chamber (HRCC) engine operating at a higher CR on high octane fuel, reported in SAE paper No. 810017, 1981.
The main limitation of using high compression ratios with gasoline fuels is engine knock at high load due to the limited octane rating of most fuels. Even with the use of high octane rating fuels such as natural gas, use of high compression ratio has been of limited success, as found by Tecogen Inc., which makes natural gas based co-generation equipment using standard 2-valve gasoline engines converted to natural gas. High CR in the preferred range of 13 to 1 to 18 to 1 by necessity produces high engine cylinder pressures which stress the engine, and with engine knock, can damage the engine. But since an engine operates over a wide range of loads in a real world vehicle, it follows that under light load conditions, where the peak compression and combustion pressures are lower, high CR can be used.
Therefore, considerable work has been done with Variable Compression Ratio (VCR) systems to achieve a high CR at light loads and a low CR and high loads. Generally, they fall into two types: mechanical linkage type, of which there are many, and oil pressurized pistons. Of the mechanical linkage type, U.S. Pat. Nos. 4,517,931 and 6,412,453 are but a sampling. Of the oil-pressurized piston type, U.S. Pat. No. 4,241,705 is an example.
Another approach, which represents an indirect form of VCR, is to use a flexible material within, or connected to the piston, that gives way to limit the peak pressures, as exemplified by U.S. Pat. No. 6,568,357 B1, which uses elastomers, and by my PCT patent application PCT/US03/12058, referred to hence forth as '058, with International Publication No. WO 03/089785 A2 and date of 30 Oct. 2003, which uses preferably metallic springs either in the engine piston or connecting rod to both limit peak pressures at high load and allow for substantial pressures on compression at light loads so that strong air-squish is present to speed up the burn of ultra-lean mixtures. The importance of squish, especially in interacting with flow-coupling ignition sparks, is disclosed in my U.S. Pat. No. 6,267,107 B1, referred to hence forth as '107. The disclosures of my published patent application '058 and patent '107, and other patents, patent applications and published articles cited below, are incorporated herein by reference as though set out at length herein.
While these address but do not exhaust the possible ways of offering VCR systems for handling the issues of engine knock at high CR in gasoline engines, none of them address in detail the more fundamental problem of the Otto cycle for achieving best efficiency and power under all engine operating conditions, from light load where lean burn, fast burn is used at high compression ratio, to high load, where stoichiometric operation, with or without EGR is used, depending on the load requirements, to achieve an engine with highest efficiency, highest power, and low emissions.
Once the problem of lean burn (fast burn) has been solved, as has been done by my company, Combustion Electromagnetics Inc., CEI, as described in an SAE paper No. 2001-01-0548, the next step is to consider higher compression ratios. In our case, this is especially important in view of the fact that in the engine tests we conducted, we found that the lean burn capability of the engine tested (using homogeneous mixtures) was better at higher CR, where it was shown that at approximately 14 to 1 CR, the lean burn capability of the engine was well over the 30 to 1 air-fuel ratio (AFR) of the 11 to 1 CR, around 36 to 1 AFR and higher, depending on CR, also disclosed in my patent application '058. It is believed that this is in part due to the higher squish and turbulence at the higher CR, as well as to the higher adiabatic heating of the ultra lean mixture, to raise it to a relatively higher gas temperature prior to ignition to partly compensate the smaller amount of fuel. That is, the leaner mixture has a lower specific heat Cv at constant volume and a higher specific heat ratio γ, where γ=Cp/Cv, and where Cp is the specific heat at constant pressure.